Abstract: The cracking failure cause of expansion pipe joints of convection pipes of a sugar refinery’s boiler was analyzed through macroscopic examination, chemical compositions analysis, SEM fracture analysis, metallographic examination and mechanical properties test. The results show that inflation and recovery repeatedly of convection pipes which suspending water drum, during the frequently quick startup and stop process of the boiler, led to the expansion pipe joints fatigue cracking by the role of largest stress concentration and the bending stress, and the cracks development led to the brittle cracking along the grain boundary.